Today, for the sake of high speed mobile communication, many wireless communication technologies have been proposed as candidates. Among these, an Orthogonal Frequency Division Multiplexing (OFDM) technique is now recognized as the most leading next-generation wireless communication technology. In the future, it is expected that the OFDM technology will be used in most of the wireless communication technologies. At present, even Institute of Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN) that is called 3.5-Generation (3.5G) technology adopts the OFDM technology as standards.
The OFDM scheme is a scheme of transmitting data using a multi-carrier. Namely, the OFDM scheme is a type of Multi Carrier Modulation (MCM) scheme of parallel converting symbol streams input in series and modulating each of the symbol streams into a plurality of sub-carriers having cross orthogonality, i.e., a plurality of sub-channels for transmission.
In a broadband wireless communication system using the OFDM scheme, a Base Station (BS) transmits a Synchronization Channel (SCH) to a Mobile Station (MS) for the sake of timing synchronization and BS distinguishment. Namely, the MS can distinguish the BS to which it belongs using the SCH. A position where the SCH is transmitted is predefined between a transmit end and a receive end. As a result, the SCH operates as a kind of reference signal.
The SCH can be designed in various methods. Among them, the most noticed method is currently a method of loading and transmitting a Pseudo-Random (PR) sequence native to a BS on subcarriers at predetermined intervals at a frequency domain. In the case of mapping a sequence at predetermined intervals without loading and transmitting a sequence on all subcarriers, regarding a time domain signal after an Inverse Fast Fourier Transform (IFFT) operation, it can be identified that a repetition of a constant pattern takes place within an OFDM symbol. Here, the repetition count is varied depending on the sequence mapping interval of the frequency domain.
An SCH used in a conventional IEEE 802.16e system is described below.
FIG. 1 illustrates an SCH of a conventional system at a frequency domain. As illustrated in FIG. 1, in the conventional SCH, a sequence value is allocated every three-subcarrier intervals at the frequency domain.
A time domain signal of an SCH corresponding to that of FIG. 1 is illustrated in FIG. 2. Referring to FIG. 2, the conventional SCH has a format in which the same signal is repeated 3 times at a time domain. An MS acquires timing synchronization using a repetition pattern of the SCH. At this time, a size of IFFT is equal to the power of ‘2’; however, the ‘3’ (repetition count) is not equal to a divisor of the IFFT size and therefore, the three-time repetition pattern is not a complete repetition pattern but an incomplete repetition pattern. Accordingly, in case that the MS is positioned at a cell boundary or cell edge of a BS, there may occur a problem that, because an SCH of an adjacent cell acts as interference, the three-time repetition pattern is broken. In this case, the MS has difficulty acquiring timing synchronization.
Additionally, the conventional SCH uses a sequence of the same length as that of the number of subcarriers allocated to one SCH. A conventional IEEE 802.16e system uses 114 sequences to distinguish the total 114 BSs. For example when a length of IFFT is equal to ‘1024’, a length of each sequence is equal to ‘284’ that is the number of subcarriers allocated to an SCH. At this time, an MS determines correlation values between a received SCH signal and the 114 sequences previously possessed, and acquires a cell IDentification (ID).
An IEEE 802.16m system, a system evolving from the conventional IEEE 802.16e system, requires more cell IDs than the IEEE 802.16e system. Also, even the number of sequences of an SCH symbol (i.e., an OFDM symbol) is increased in proportion to the number of cell IDs. The increase of the number of sequences may result in a deterioration of a correlation characteristic between sequences and a degradation of cell ID detection performance and also, an increase of a Peak to Average Power Ratio (PAPR) of the sequence and a decrease of a margin capable of boosting a transmit power of an SCH.
Also, the IEEE 802.16m system can require that an SCH include supplementary information (i.e., a system parameter) other than cell ID information. As such, an SCH of a future system (e.g., IEEE 802.16m) should be newly designed to meet additional requirements of many cell IDs and supplementary information transmission, and so forth. At this time, it is required to optimally design sequences of the SCH in consideration of cross correlation characteristics and PAPRs.